1. Technical of the Field
The invention relates to a relay unit which include two-system switches that relay frames and which is cascaded in multiple stages to form a LAN.
2. Description of the Related Art
For example, in a communication network such as a layer 2 network, a structure has been used in which plural switches are connected to each other in a ring shape or a mesh shape in order to achieve redundancy with respect to the relay of frames. When this structure is simply used as is, frames are continuously looped between the switches. As a technique for preventing an infinite loop of frames, a spanning tree technique is known, for example. This technique determines a root switch based on an ID exchanged among the switches and determines a port to be invalidated based on a path cost from the root switch, thereby forming a loop-free tree structure. In addition, for example, Japanese Patent Application Laid-Open (JP-A) No. 2007-174119 discloses a technique for preventing a loop. JP-A No. 2007-174119 discloses a technique that arranges switches in a ring shape, and blocks the relay of frames using a specific port when a master device, which is one of the switches, has detected a loop, thereby preventing looping of frames.
However, in the spanning tree structure, since one switch is a root switch, the position at which the relay of frames is blocked is fixed and it is difficult to flexibly switch the relay routes when a failure occurs. As a result, it is difficult to effectively use the relay paths that can be formed between the switches. In addition, in JP-A No. 2007-174119, a loop is detected using the master device as the base, and the master device blocks the relay of frames using a specific port. Therefore, a similar situation occurs. In these techniques, when relay units including the switches are cascaded in multiple stages, it is thought that it is difficult to flexibly change the switching aspect of the relay route according to the number of connections.